1. Field of the Invention
The invention relates to a method for controlling a piezoelectric actuator which is used to displace an element. With a method of this kind the actuator has a control voltage applied to it. Because of the change in length of the actuator induced by the activation the element is displaced.
2. Description of the Prior Art
Piezoelectric actuators are used for example in fuel injection systems. In this case the element is a control valve which is operated by the actuator.
Since the length of the piezoelectric actuator is dependent on temperature, a safety gap, referred to as the return stroke, should exist between the actuator in the non-activated state and the element, said gap ensuring that even in the event of temperature changes the non-activated actuator does not displace the element.
Efforts are being made to make the size of the return stroke independent of ambient influences, as otherwise the element will be displaced more or less forcefully or in the extreme case not all when the actuator is activated. With a fuel injector, a change in the return stroke effects for example a delay in the start of injection and the end of injection, which can result in high emissions and loud combustion noises.
It is known that the influence of temperature fluctuations on the return stroke can be diminished if elements which determine the size of the return stroke are made of materials with coordinated thermal coefficients of expansion. Invar, for example, has revealed itself to be a suitable material for a housing for the piezoelectric actuator.
However, compensating for the temperature-induced expansion of the actuator does not entirely succeed by this measure, since not all the components surrounding the actuator can be made of invar. Moreover, the measure does not help in the case of changes in the return stroke caused by ambient influences that are not temperature-induced.